For BGRX, the alpha value is undefined. It wont write back to client
memory, so don't compare alpha to expected result.

Bug: angleproject:5958
Change-Id: I289f14f7c6a6ee63e74351d6abdd44fff8bf106b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2891926Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>